The present invention relates in general to photolithography and, more particularly to a multi-tone photomask and method for manufacturing the same.
As semiconductor device manufacturers continue to produce smaller devices, the requirements of photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically include substrates (e.g., high-purity quartz or glass) that have an absorber layer (e.g., chrome) formed on a substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semi-conductor wafers in a lithography process. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
In some semiconductor fabrication processes, a special photomask may be used to image a pattern on to a surface. One example of a special photomask is a tri-tone photomask. A conventional tri-tone mask typically includes at least two material layers formed on a substrate, where each type of material transmits a different percentage of an exposure wavelength from a lithography system.
A tri-tone photomask may be manufactured using a conventional additive process. In a conventional additive process, a binary photomask may be formed by imaging a pattern into a resist layer of a photomask blank, developing the resist layer, etching exposed portions of the absorber layer and stripping the remaining resist layer. Another pattern may then be formed on the photomask blank by depositing another resist layer, imaging the pattern in the resist layer, developing the resist layer and etching exposed portions of the absorber layer. Before the remaining resist layer is removed, the photomask blank may be sent to a third party supplier to deposit a filter layer of partially transmissive material. The remaining resist layer under the filter layer is then removed to form the tri-tone photomask.
Numerous problems may occur during the fabrication process. Typically, the partially transmissive material is not deposited uniformly over the surface of the photomask because of the different surface elevations on the photomask. For example, the thickness of the partially transmissive material may vary or be inconsistent near the edges of features. These variations in thickness and inconsistencies near the edges of the features may cause light scattering or otherwise affect reflectivity, which can corrupt the projected image. The thicker part of the partially transmissive material may also be fragile and susceptible to degradation, which can cause feature uniformity problems at the interface of the absorber layer and partially transmissive material.
Problems may also occur because the photomask must be sent to a third party supplier in the middle of the fabrication process. Defects may be caused by the extra handling that occurs when the photomask is sent to the third party supplier. Furthermore, the amount of time that is needed by a photomask manufacturer to fabricate the tri-tone mask is increased because the manufacturer must send the mask to a third party supplier.
Problems may also continue when the photomask is used in the manufacturing process. Because the partially transmissive material is exposed, it may undergo oxidation that may change the transmissive properties of the material. This may result in a reduction of the usable life span of the photomask.
Therefore, a need has arisen for a method for manufacturing a multi-tone photomask which does not require the photomask to be sent to a third party supplier to add a filter layer during the manufacturing process.
A further need exists for a photomask which has pre-deposited filter layer that meets optical specifications provided by a manufacturer.
A further need exists for a photomask which provides a protective coating on a filter layer such that the filter layer does not oxidize over time.
In accordance with teachings of the present invention, disadvantages and problems associated with manufacturing a multi-tone photomask have been substantially reduced or eliminated. A barrier layer located between an absorber layer and a filter layer on a photomask acts as an etch stop for the absorber layer and provides a protective coating to prevent the filter layer from being oxidized over the lifetime of the photomask.
More specifically, a filter layer formed on at least a portion of substrate includes a first pattern created by using a wet etch process. The absorber layer further includes a first etch characteristic. A barrier layer formed on at least a portion of the filter layer has a second etch characteristic and provides a protective coating that prevents the filter layer from oxidizing over time. The barrier layer includes the first pattern formed by a dry etch process. An absorber layer that includes a second pattern formed by the wet etch process is formed on at least a portion of the barrier layer. The barrier layer is inert to the wet etch process and further provides an etch stop for the wet etch process.
In one embodiment, a photomask includes a filter layer formed on at least a portion of a substrate, a barrier layer formed on at least a portion of the filter layer and an absorber layer formed on at least a portion of the barrier layer. The filter layer includes a first pattern formed by a first etch process and the barrier layer includes the first pattern formed by a second etch process. The absorber layer includes a second pattern formed by a third etch process. The barrier layer provides an etch stop for the third etch process.
In another embodiment, the absorber layer and the filter layer include a first etch characteristic such that exposed portions of the layers are removed by wet etch processes. The barrier layer includes a second etch characteristic such that exposed portions of the barrier layer are inert to the wet etch process and the exposed portions are removed by a dry etch process. The barrier layer further provides a protective coating to prevent the filter layer from oxidizing.
In a further embodiment, the filter layer is a neutral density material that transmits less than approximately fifty percent of radiant energy and the absorber layer transmits approximately zero percent of radiant energy. The barrier layer is silicon dioxide or aluminum oxide and includes a thickness approximately equal to one quarter of an exposure wavelength.
In one embodiment of the present invention, a photomask includes a filter layer with a first etch characteristic and a first transmittance. A barrier layer with a second etch characteristic is formed on the filter layer. An absorber layer is formed on the barrier layer. The absorber layer includes the first etch characteristic and a second transmittance. The second etch characteristic of the barrier layer is inert to the etch processes used on the absorber and filter layers.
In an additional embodiment of the present invention, a method for manufacturing a photomask includes providing a photomask with a barrier layer located between an absorber layer and filter layer. A first pattern is formed on the absorber layer using a first etch process. The first etch process is stopped by the barrier layer. A second pattern is formed on the barrier layer by a second etch process. The second pattern is formed in the filter layer by using a third etch process.
Important technical advantages of certain embodiments of the present invention include a photomask blank that decreases the time needed to fabricate a multi-tone photomask. The blank includes a pre-deposited filter layer formed on a substrate. Since the blank includes the filter layer, a photomask manufacturer does not have to interrupt a photomask manufacturing process to send the partially fabricated photomask to a third party supplier. Furthermore, the throughput and yield may increase since the extra handling steps associated with transporting the photomask between manufacturing facilities is eliminated.
Another important technical advantages of certain embodiments of the present invention includes a filter layer that may be optically measured and qualified before beginning a photomask fabrication process. Since the filter layer is deposited before a photomask fabrication process begins, the filter layer may be deposited according to specifications provided by a photomask manufacturer or a semiconductor manufacturer. The filter layer, therefore, may have a uniform thickness across the surface of a photomask. The filter layer may also be varied in composition to meet optical specifications of the manufacturer.
A further important technical advantage of certain embodiments of the present invention includes a barrier layer that prevents optical characteristics associated with a filter layer from changing over time. The barrier layer is formed on at least a portion of the filter layer and provides a protective coating that effectively seals areas of the filter layer not covered by an absorber layer. The barrier layer, therefore, protects the filter layer during a manufacturing process and prevents oxidation from altering the optical characteristics of the filter layer.